Integrated semiconductor optical devices have been known in which an optical active device and an optical modulator are integrated on a substrate, as disclosed in, e.g., Japanese Laid-Open Patent Publication 10-173291 (1998). It is generally preferred that the plurality of integrated components in an integrated semiconductor optical device be electrically separated from one another. In this connection, the integrated semiconductor optical device disclosed in the above publication includes a separating trench formed between the semiconductor laser and the modulator. A semi-insulating semiconductor layer is formed buried in this separating trench. In this way the semiconductor laser is electrically separated from the optical modulator.
Other prior art includes Japanese Laid-Open Patent Publication Nos. 7-7226 (1995), 63-186210 (1988), 62-219590 (1987), 5-226633 (1993), 1-192184 (1989), and 59-129473 (1984).
There is the type of integrated semiconductor optical device which includes an integrated optical active device and an integrated optical waveguide. The optical waveguide includes an optical guiding layer and two cladding layer sandwiching the optical guiding layer therebetween. It will be noted that in an integrated structure including an optical active device and an optical waveguide, if a side of a cladding layer of the optical waveguide is in contact with the optical active device, a current path may be formed between the optical active device and the optical waveguide side through that cladding layer. If a current leaks from the optical active device to the optical waveguide side through this current path, a degradation in the characteristics of the optical active device may result.